1. Field of the Invention
The invention relates to a surface treatment of metal products, preferably sheets, especially of bare steel products or steel products coated with a metal plating. The treated metal products also make up part of the invention as does their use.
2. Discussion of the Background
Surface treatments are known in at least three general areas: those that are used to prepare a metal surface for the deposition of a coat (such as chromating), to protect against corrosion (such as oiling) and to lubricate the surface for forming or deep-drawing operations (also such as oiling).
The first area is that of preparation of a surface for deposition of a coating.
To make food packagings, there are currently used tin-plated sheets (tin- plate) coated with a layer of protective varnish. Application of the protective varnish necessitates preliminary treatment of the metal surface, in particular to achieve good adherence of the varnish to the surface under conditions that include aqueous, solvent or corrosive environments. This preliminary treatment of the metal surface currently consists of a passivation treatment of the chemical or electrochemical chromating type. The use of such treatments generally involves risks of corrosion and pollution related to chromium, especially during handling of treatment solutions and disposal of treatment effluents.
U.S. Pat. No. 5,103,550 describes a process for the application of varnish on the surface of an aluminum metal sheet wherein:
there is performed a preliminary surface treatment consisting of: PA1 and there is immediately applied to the treated surface a varnish, for example a modified epoxy polyvinyl chloride varnish of the type commonly used in the area of sheets for foodstuff packaging. PA1 the surface is treated by application of a homogeneous monomolecular layer of an organic product (or "coupling agent") based on molecules, the main skeleton of which has the form of a linear carbon chain, containing a polar siloxane radical at one end only, PA1 the said fluorocarbon polymer is applied immediately, PA1 then the polymer is made to react on the said film and the polymer is made to undergo cross-linking, especially by irradiation. PA1 there is applied to the surface of a metal product a film of an organic product based on molecules, the main skeleton of which has the form of an apolar linear chain consisting of saturated carbon-carbon bonds at one end and containing a polar radical at the other end only, PA1 the film is heated to a suitable temperature for a suitable time so that all or a portion of the organic product becomes fixed on the metal surface and/or the organic product becomes partly cross-linked on the metal surface, PA1 and that, optionally, any quantity of organic product which is not fixed and/or cross-linked to the said surface is then eliminated from the film, if necessary, by, e.g., using a solvent for the organic product. PA1 the conditions for application and heating of the said film are adapted such that, after treatment, the proportion of the polar radicals on the film surface is higher than the stoichiometric proportion of these radicals in the molecules of the said organic product. PA1 the polar radical is chosen from among at least the following radicals: carboxylic acid, phenol, epoxy, vinyl, amine or silane. PA1 the thickness of the film comprises at least two monomolecular layers of the said organic product. PA1 the film is homogeneous. PA1 in the case of a metal-plated steel sheet, the metal plating comprises or consists of zinc, zinc alloy, chromium or chromium alloy, tin or tin alloy. PA1 in a first stage, the metal surface is treated according to the invention method set out above (film of organic product, etc.), PA1 then, in a second stage, the varnish is applied directly on the treated metal surface. PA1 the period between the first stage and the second stage is longer than 1 day. PA1 the varnish is of the polar type. PA1 in a first stage, at least that surface of the sheet which will come into contact with the forming tool is surface treated according to one or more of the processes of the invention, PA1 and, in a second stage, the actual forming process is carried out by means of the said tool.
a) anodizing the said surface to form an oxidized surface, PA2 b) applying a homogeneous film of organic product based on phosphonic or phosphinic acids or esters or on salts thereof, which substances may or may not be polymerized, PA2 c) then eliminating from the said film, by rinsing and if necessary drying the said film, that quantity of organic product which is not fixed to the said surface, if necessary, by means of a solvent for the said organic product,
The surface treatment described in this document advantageously replaces a chromating treatment for promoting adherence of the varnish to the sheet. However, this surface treatment necessitates a preliminary anodization stage, which complicates the process; in addition, this stage is not necessarily feasible for other types of substrates, such as on steel sheets coated with a metallic plating, for example tin-plated, chrome-plated or zinc-plated steel sheets. In addition, this surface treatment necessitates immediate application of the varnish, since the treated surface cannot retain its properties that favor adherence of the varnish. Such a surface treatment therefore does not permit plans to store the treated sheets, which is a serious drawback.
Document EP 484,886 describes a process for application of a fluorocarbon polymer on the surface of a metal sheet, especially of aluminum, wherein:
According to this document, the reaction that induces adherence between the film of coupling agent and the applied polymer layer assumes C.dbd.C or C.tbd.C bonds, especially in the linear carbon chain at the end opposite from that of the siloxane radical. Thus the drawback of such a process is that it necessitates application of an exclusively monomolecular film of coupling agent, which is sometimes difficult to achieve by simple methods. Another drawback of such a process is that it necessitates a reaction stage, for example by means of irradiation, to make the polymer layer react with and adhere to the surface-treatment film. A further drawback of such a process is that a treated surface cannot be stored without losing its "coupling" properties, since the C.dbd.C or C.tbd.C bonds become progressively oxidized, especially due to natural exposure to ultraviolet radiation.
The second and third areas are those of corrosion protection and lubrication for forming, especially deep-drawing.
On discharge from fabrication, sheets are generally oiled to provide corrosion protection at least temporarily, or in other words for at least a storage period (at least one day, and sometimes even several months).
The oils used for such protection must also be adapted to facilitate forming, without prior degreasing, immediately on removal from storage.
Oils commonly used for such applications are, for example, based on dioctyl sebacate or acetyl tributyl citrate.
Thus sheets to be coated are generally oiled when removed from storage, and so it is advisable to degrease them first of all. This means that at least three operations--oiling, degreasing, chromating--can be identified between discharge of the sheets from fabrication and admission to the coating installation (varnishing or painting, for example).
In addition, despite the degreasing operation prior to the coating process, "varnish-repelling effects" are frequently observed during application of the varnish, thus leading to the presence of zones not covered by the varnish on the sheet surface.
These varnish-repelling effects generally result from defects and difficulties during degreasing, and may lead to high reject ratios.